Daz, any clever ways of getting a sprocket off when it is really really tight? I can't get the one on the disc wheel off

I also couldn't get the one on the bike I am selling off either, but the guy wanting to buy it did... we put a 14 on the back, in stead of the 15 that is stuck on the disc... He rode on 49/14 :-O and didn't seem to mind, so am selling the bike with that gear on now and will buy myself a new 14 sprocket...